The present invention relates generally to the use of electric pulses discharged between submerged electrodes for repelling fishes. For example, it relates to the use of electric fish barriers for stopping or guiding the passage of fishes through a natural or artificial waterway such as during seasonal upstream and downstream migrations. Other uses of such apparatus include the repelling of fishes away from water outlets such as drains, hydroelectric generator intakes, flumes, spillways and other dangerous areas into which they might otherwise reach or be drawn into by water flow.
One of the most desirable characteristics of a successful electric fish barrier is one where fishes entering the repelling zone experience a gradually increasing unpleasant sensation as they advance further into the electrified area. The electric zone frighten the fishes away from the barrier as efficiently as possible without causing them injury. The successful electric fish barrier described in the present specification allows fishes to learn to avoid the barrier and thus safely bypass potential danger areas.
The criterion essential for electric barriers to block upstream fish migration requires only that fishes receive enough electric current to reduce their swimming ability to a level below which they are unable to maintain progress against the stream water flow. Both alternating current (A.C.) and direct current (D.C.) can be used for upstream fish barriers. However, electric pulses derived from A.C., both single and multiphase, are considered more stressful to fishes than D.C., and therefore are more likely to cause them serious injury. Half-wave and full-wave unidirectional (D.C.) pulses are more commonly used to block fish migrations because pulsed D.C., while tetanizing, does not usually produce severe physiological effects in fishes as does A.C. and, in the case of upstream migrants, the immobilized fishes are swept clear of the electrified zone by stream water flow.
Alternatively, when electric fish barriers are operated to block downstream migration of fishes it is essential to avoid tetanus, or otherwise reduce the swimming ability of fishes, which would allow them to be swept further into the electrified area by water flow. To avoid the tetanizing effect, short duration, unidirectional pulses can be used to advantage.
In both upstream and downstream electric fish barrier designs it is important that the voltage gradient within the electrified zone gradually increase from a level below the threshold needed to repel fishes, to a level above. This type of electric fish barrier allows fishes to learn to taken an alternate path, and thus be guided around potential danger areas.
In the past such apparatus has consisted of at least one submerged active electrode from which fishes would be repelled; at least one return electrode, either submerged or otherwise earth connected, to complete the circuit; and a suitable electric power source for discharging electric current between the active and return electrodes. Although generally, in the past, a plurality of active electrodes were deployed in an attempt to adequately distribute the electric field for the purposes mentioned, they were either connected in parallel to produce a single dipole electric field, or otherwise deployed and connected to produce a plurality of separate dipole electric fields in an attempt to optimize the fish repelling function.
Electric fish barriers used in the past were characterized by repelling zones too narrowly confined to the volume of water immediately adjacent the electrodes, allowing fishes to approach to close before being adequately stimulated to produce the desired fright response. However, when the electric output used in such fish barriers was increased enough to substantially widen the repelling zone, the current density would often become harmful or lethal to fishes in the area immediately adjacent the electrodes. For example, U.S. Pat. No. 1,515,547 (Burkey); U.S. Pat. No. 2,010,601 (Loughbridge); U.S. Pat. No. 4,593,648 (Marzluf) illustrate those structural features and limitations which characterize electric fish barriers used in the past.